1. Field of the Invention
The present invention relates to a compound semiconductor device and, more particularly, to a compound semiconductor device using a two-dimensional electron gas.
2. Description of the Prior Art
Hitherto, as a compound semiconductor device using a two-dimensional electron gas (2DEG), a high electron mobility transistor (HEMT) has been known. In the HEMT, the 2DEG which is formed in the channel layer near the heterointerface between the channel layer and the electron supply layer is used as a channel. An n-type AlGaAs/GaAs HEMT using an n-type AlGaAs layer and an undoped GaAs layer as an electron supply layer and a channel layer, respectively has been most well-known.
In recent years, in the n-type AlGaAs/GaAs HEMT, there has been proposed a method whereby the energy difference .DELTA. Ec of the conduction band edge (Ec) at the n-type AlGaAs/GaAs heterointerface is increased by introducing an InGaAs layer of constant compositions whose electron affinity is larger than that of GaAs between the n-type AlGaAs layer and the GaAs layer, thereby increasing the 2DEG concentration n.sub.s. In this case, since the InGaAs lattice constant is larger than that of GaAs, the InGaAs layer is lattice-mismatched to the GaAs layer. Thus, a lattice distortion occurs in the InGaAs layer. For this reason, the InGaAs layer is called a pseudomorphic (or strained) layer. When the thickness of InGaAs layer is set to a value called the critical layer thickness which is determined by the composition of this layer or more, misfit dislocations occur at the interface between the InGaAs layer and the GaAs layer. Therefore, to suppress the occurrence of such misfit dislocations, it is necessary to set the thickness of InGaAs layer to the critical layer thickness or less.
To improve the characteristics of the HEMT, it is necessary to increase the 2DEG concentration n.sub.s and the saturation velocity v.sub.s of electrons. In the case where the InGaAs layer having constant compositions is used as a pseudomorphic layer as mentioned above, it is advantageous because as the In composition is large, the 2DEG concentration n.sub.s and the saturation velocity v.sub.s increase. However, there are the following problems (Jpn. J. Appl. Phys. 26 (1987) 539).
(1) As the In composition is large, the critical layer thickness decreases. For instance, it has been known that when the In composition is set to 20%, the critical layer thickness is 200 .ANG. or less.
(2) When the thickness of InGaAs layer reaches a value (&lt;200 .ANG.) such as to cause the quantum effect as the result of that the thickness of InGaAs layer was set to a value which is equal to or less than the critical layer thickness, energy levels rise. Thus, even if the In composition increases, the 2DEG concentration n.sub.s is not increased remarkably.
Therefore, in the conventional n-type AlGaAs/InGaAs/GaAs HEMT using the InGaAs layer as a pseudomorphic layer, when the 2DEG concentration n.sub.s is intended to be maximized, the In composition of the InGaAs layer is determined to a value within a range from 15 to 20% and the thickness is determined to a value within a range from 200 to 150 .ANG.. Therefore, degrees of freedom in selection of the In composition and the InGaAs thickness are small. On the other hand, in the n-type AlGaAs/InGaAs/GaAs HEMT, electrons are confined to the quantum well formed by the double heterojunctions comprising the n-type AlGaAs/InGaAs heterojunction and the InGaAs/GaAs heterojunction and thereby the 2DEG is formed. Therefore, in the case where defects such as alloy clusters or the like and other scatterer exist in the double heterojunctions, the characteristics of the HEMT can be easily deteriorated due to influences by them.
Further, when the HEMT is fabricated by molecular beam epitaxy (MBE), the growth temperature of the InGaAs layer needs to be set to be lower than the growth temperature of the GaAs layer or the AlGaAs layer by about 100.degree. C. For this purpose, for instance, after the GaAs layer was grown, it is necessary to immediately lower the growth temperature by about 100.degree. C. and to grow the InGaAs layer. However, it is not so easy to suddenly change the growth temperature with good controllability. If the controllability of the growth temperature is bad, there is a fear such that alloy clusters or the like are formed at the heterointerfaces of the double heterojunctions and characteristics of the HEMT deteriorate. Therefore, to prevent such a problem, enough attention must be paid to the control of the growth temperature. On the other hand, since the thickness of InGaAs layer is so small to be 200 to 150 .ANG. as mentioned above, it is not so easy to grow the InGaAs layer with good controllability of the thickness.